Most power systems employ more than one electrical generator to supply the required load. Each generator is switched in or out of the system to meet the demand load.
Before the incoming generator can be paralleled to the bus bars the following conditions must be fulfilled:
(i) The machine voltage must be equal to the system voltage. PA1 (ii) The phase of the generator voltage must be in substantial coincidence with that of the system. PA1 (iii) The frequency of the generator voltage must be the same as that of the system.
In practice these conditions are only approximately adhered to. The operator brings the incoming generator voltage, phase and frequency up to the required limit using a voltmeter and synchrosope and switches in the alternator when all three factors are satisfied. However, as a safety measure, a check synchroniser is also linked into the system to ensure that conditions are correct before the circuit breaker is closed.
In its simplest form, check synchroniser is a piece of apparatus which determines when the voltage phase and frequency are within safe limits for paralleling. The output relay contacts of the unit are connected in series with a no-volts coil so that, unless its contacts are closed, the circuit breaker cannot be closed.